1. Field of the Invention
This invention relates to carrier tapes for semiconductor devices. More particularly, this invention relates to patterns of lead clusters in carrier tapes and methods of bonding chips to lead clusters. The invention further relates to certain deformations of the leads in the clusters so that each chip, after it is bonded to the leads, maintains a given posture in the assembled device. Also, the invention relates to certain flexural and tensile stresses developed in each lead during deformation and the control of such stresses.
2. Prior Art
AUTOMATION IN THE SEMICONDUCTOR INDUSTRY
There is a growing need in the industry to lower prices of assembled semiconductor devices. These lower prices are best obtained by automating the assembly of the semiconductor devices.
Automation is enhanced by continuous and uninterrupted movement of the components that go into making the ultimate semiconductor device. But the components are very small and in most cases very fragile. Thus they are difficult to handle efficiently.
One solution to the handling problem is to affix the components to a constantly moving web of material. This web of material, generally called a carrier tape, is used to carry the components as they are formed into subassemblies. The tape performs the further function of providing parts such as leads to the devices.
CARRIER TAPE
The industry is now developing these carrier tapes to automate the assembly of the devices. These tapes are passed over a plate for carrying a multitude of semiconductor chips. The tape is also indexed a given distance at a time over various types of bonding machines. A chip is picked up from its supporting plate after it is bonded to the tape at each indexing cycle.
The chips are bonded to clusters leads. These leads are either fabricated out of the tape itself or they are fabricated out of material adhered to a tape substrate. The tape leads are called inner leads because each lead is later bonded to an outer lead. The outer leads are then inserted into the board of a circuit pack.
The lead clusters are incrementally spaced in the tape to coincide with each space in the indexing cycle as the tape progresses. After the chip is bonded to the cluster of inner leads, it is carried by the tape for further device assembly steps.
TAPE DEVELOPMENT
All carrier tapes include conductive material such as copper sheet out of which the leads are formed. Preferably the copper sheet is very thin to provide good bonding. Also very thin leads help to tolerate certain thermal cycling tests which must be endured by various types of assembled devices.
The copper sheet, called foil, is typically 0.7 to 2.8 mils in thickness. The foil weighs approximately 3/4 to 21/2 ounces per square foot. Preferably the material is annealled copper of "dead soft" ductility. Such foil is very delicate and requires careful handling.
A prior art way to overcome the handling problem is to use a laminated web for the tape. The main part of the web consists of a film pf polyimide plastic used as a tape substrate. A strip of conductive foil is then adhered to the entire surface of the polyimide film or to the center alone for supplying the leads for such device. Metal deposition techniques are also used to provide the strip of metal conductor.
Some manufacturers are now developing methods of using the conductive foil along, without the polyimide substrate. Such methods must avoid warping the foil while it is being wound upon reels and while it is being indexed for device assembly. The accuracy of lead fabrication must be maintained. And the leads of each lead cluster must be bonded to the chip without distorting adjacent lead clusters on the foil tape.
BONDING
The inner leads are fabricated within the foil of the carrier tape in a cluster. These leads look like fingers which project toward the center of the cluster. The ends of the leads which project toward the center are free and the opposite ends are affixed to the tape.
The free ends of the leads are simultaneously bonded to pads on the chip by thermocompression methods. Each lead is precisely registered with its bonding pad on the chip. The pad is actually a thickened terminal which is part of the electronic circuit on the chip.
All free ends of the leads in the cluster are bonded at once to the chip. This takes place with one stroke of the bonding tool in a method often called "gang bonding."
OUTER LEAD BONDING
The inner leads are too fragile for end use so they must be bonded to stiff outer leads. Such outer leads are then connected to the outside world.
The stiff outer leads are formed in a strip of metal in clusters much like the clusters of the inner leads. However, there is a much larger hole in the center of the outer lead cluster and the strip is of thicker metal. This strip is called a lead frame by those skilled in the art. Once the outer lead bonding takes place, the lead frame will carry the device for future assembly.
The outer lead bonding is achieved by machine coordinated movement of the lead frame strip, the carrier tape, and the bonding tool. The carrier tape is indexed under the lead frame strip and over the cluster punching tool.
The punching tool drives upward stressing the leads and shearing the inner lead cluster at the perimeter of the cluster where each lead is affixed to the tape. The tool carries the chip and the sheared inner lead cluster upward and out of the carrier tape. The chip is carried into the hole in the outer lead cluster and the sheared end of each tiny inner lead and each outer lead is then precisely registered one with the other. And the chip has a defined posture in the device after bonding.
Each inner lead is simultaneously bonded to its proper outer lead thus transferring the device to the outer lead frame. And the carrier tape becomes a skeleton which is then scrapped.
INNER LEAD DEFORMATION "BUGGING"
At inner lead bonding, another important step takes place in the assembly. The bonding tool uniformly deforms the soft inner leads out of the plane of the carrier strip. Such deformation takes place when the chip is bonded to the cluster of inner leads and the chip is pulled off its supporting fixture where it was adhesively mounted. The step of uniformly deforming the inner leads is called "bugging" by those skilled in the art. The term "bugging" reflects the appearance of the chip and the inner lead cluster after inner lead bonding is done properly.
If the bugging is not done properly, further assembly of the device is impaired and certain thermal cycling qualities of the device can be adversely affected. When the "bug" is made the leads are stressed and deformed so that each lead becomes a part of a 4-cornered, bell-like configuration. The fixed end of each lead remains attached to the tape and the free end of each lead is bonded to the chip. The chip rides on the top of the bell in a horizontal position precisely in the center of the lead cluster. The chip is oriented so that the edges of the chip are very closely parallel to the sides of the tape.
Proper bugging is made easier by proper layout of the lead pattern. Ideally leads should be symmetrical about any axis drawn through the center of the lead pattern. Otherwise there will be uneven development of stresses in the leads and control of the stresses will be difficult to maintain.
EFFECTS OF IMPROPER STRESS CONTROL IN THE LEADS
If the stresses in the inner lead cluster are not controlled during bugging, the carrier tape itself can be affected. The stresses can be transmitted to and warp the structural margins of the tape. These margins have sprocket holes used for indexing. And the holes are used to precisely advance the carrier tape during assembly. If the stresses warp the tape, the sprocket holes will be enlarged, thus the registration of inner leads to outer leads at the outer lead bonding will be affected.
Stresses along the centerline of the tape can warp the tape and distort the adjacent inner lead clusters on the tape. Such distortion can effect the lead-to-pad registration at chip bonding and tilt the chip on the bug. Tilting of the chip risks short circuiting between the inner leads and the edges of the chip after further assembly steps.
If the lead shift off the chip pads by just 2 mils, assembly is impaired. The actual bonding area common to the pad and its mating lead is greatly reduced. And bonds made under these circumstances are weakened substantially. Also if leads are shifted 2 mils or more, short circuiting can develop. And certain parameters such as parasitic capacitance can become a factor in reducing circuit performance.
Controlling the lead deformation improves the posture of the chip after bonding. Good posture of the chip requires that the chip not be tilted and not be off center in the lead cluster. Good chip posture assures accurate registration at outer lead bonding. Adjustments in registration by a bonding operator are eliminated. Bonding is speeded and yields are improved.
Heretofore, control of the stresses in the deformation or bugging step has been difficult and uncertain.